/Engine/Source/Runtime/Core/Public/Serialization/CompactBinaryPackage.h
An attachment is serialized as a sequence of compact binary fields with no name. It is invalid to serialize a null attachment. A raw binary attachment is serialized as an empty Binary field when empty, and otherwise as a BinaryAttachment field containing the raw hash, followed by the raw binary data in a Binary field. A compressed binary attachment is serialized as Binary with a compressed buffer as the value. An object is serialized as an empty Object field when empty, and otherwise as an ObjectAttachment field containing the hash, followed by the object data in an Object field.
template<typename BufferType, decltype(FBinaryValue(DeclVal< BufferType >().MakeOwned()))*> FCbAttachment ( BufferType && InValue, const [FIoHash](API\Runtime\Core\IO\FIoHash) & InHash )